Tenofovir (TFV) and emtricitabine (FTC) are under study in combination for HIV prophylaxis. TFV and FTC belong to the nucleoside analog reverse transcriptase inhibitors (NRTls) drug class, which undergoes sequential phosphorylation in patients to the intracellular triphosphate anabolite. The pharmacologic activity of NRTls depends on the triphosphate, which inhibits HIV reverse transcriptase to elicit pharmacologic effect. The phosphorylated anabolites are ionized and trapped within cells, which leads to differing pharmacokinetics compared with the parent drug in plasma. The dosing of NRTls in patients is guided by the intracellular profile of the active triphosphates, which demonstrates the human health importance of NRTl cellular pharmacology information. HIV prophylaxis is a new use for TFV and FTC in a new patient group (HIV-negative persons). Cellular pharmacology data are needed to support and guide this new use, but this information has not been generated. It is not acceptable to extrapolate intracellular information from HIV positive patients under treatment for chronic HIV-infection because this is a different patient group and treatment scenario. In fact, the patient groups have different cellular activation states and different pharmacodynamic considerations exist for prophylaxis compared with treatment of chronic HIV. The studies proposed herein will provide cellular pharmacology data relevant to HIV prophylaxis. The main goals are: to characterize and compare the intracellular profiles of TFV and FTC in HIV-negative versus HIV-positive adults; to identify a prophylactic threshold for intracellular TFV and FTC in HIV-negative adults; to develop a comprehensive pharmacokinetic model for intracellular TFV and FTC to predict the optimal dose and onset and duration of prophylactic action; and to evaluate whether the pharmacologic spectrum of TFV includes inhibition of purine nucleoside phosphorylase in vivo. The application addresses our long term goal to promote the optimal use of TFV and FTC for HIV prevention and treatment through rational dosing strategies founded on TFV and FTC cellular pharmacology in patients.